Devices of focusing of electronic beams



Sept. 24, 1957 R. BERTEROTTIERE ETAL DEVICES OF FOCUSING OF ELECTRONIC BEAMS Filed July 26, 1951 3 Sheets-Sheet 1 ,RENE MiRTERoTTIERH Sept. 1957 R. BERTEROTTIERE ETAL DEVICES OF FOCUSING OF ELECTRONIC BEAMS Filed Jul 26, 1951 3 Sheets-Sheet 2 K8 BERTEROTTIERE 4-,

' GU) C NVERT .IA/l/EMTMS AGENTS Sept. 24, 1957 R. BERTEROTTIERE ETA!- 2,807,739

DEVICES OF FOCUSING OF ELECTRONIC BEAMS Filed July 26, 1951 3 Sheets-Sheet 5 RENE ZBERTERQTTIERE 1* United States Patent Chico 7 2,807,739 Patented Sept. 24, 1957 DEVICES F FOCUSING OF ELECTRONIC BEAIWS 5 Ren Berterotliere and Guy Convert, Paris, France, as-

signors .to Compagnie Generale dc Telegraphic Sans Fil, a corporation of France Application July 26, 1951, Serial No. 238,594 q Claims priority, application France August 12, 1950 6 Claims. Cl. 313-82) sive'surface of the cathode, focussing of the beam being effected in a direction perpendicular to the electric and magnetic fields.

Such beams are used, for example, in travelling wave tubes with transverse magnetic field. In' these tubes it is desirable to obtain very thin electron beams carrying an intense current between two conducting surfaces maintained at two different potentials, the beam being stabilized due to the presence of a uniform magnetic field, parallel to the said surfaces and perpendicular to the beam.

'The object of the present invention is to provide an electron gun, capable of generating such beams, from a 'large,'equipotential cathode and in the presence of a uniform magnetic field. Devices capable of directing small current beams into the interaction space of a travelling wave tube do not seriously involve taking into account, in the design of such devices, the elfects of space charge. The present invention enableshigh current, i. e. intense beams to be used and takes into account the effects of space charge. As will be seen later, the efiects of space charge become more and more important as the electronic density is increased. Moreover, if the electronicemission is limited by space charge effects, it becomes more stable and Schot-tky effect is decreased.

According to the invention an electron discharge device includes a tube having two conductive electrodes of nonmagnetic material, whose surfaces are equidistant from each other over at least part of the length thereof in a predetermined plane, and means for directing a magnetic field through the space between said conductive electrodes in the direction perpendicular to said plane, said tube comprising further an emissive cathode arranged and situated to emit a space-charge limited beam into said space, curved focussing electrodes of non-magnetic material havingssurfaces whose generatrices are parallel to the surfaces of said conductive electrodes, means for applying potentials to said two conductive electrodes so as to create an electric field in the space therebetween perpendicular to said magnetic field, and means for applying potential to said focussing electrodes so that the last-mentioned potentials increase with increased distance of said focussing electrodes from said cathode, and within a range not lower than that applied to the conductive electrode having the higher potential, said potentials and the shapes of ,said electrodes being such :as to make the potential and normal potential gradient at the edge of the space charge limited beam of electrons emitted from said cathode and directed into said space the same as it would be in a space charge limited beam of infinite cross section.

The design of a gun in accordance with this invention takes as its starting point the study of the plane diode with magnetic field, by a method similar to the one used by Pierce for the diode. (Pierce, Rectilinear electron flow in beams. Journal of Applied Physics 1940vol. VIN0. 8, pp. 548-554.)

Such a diode is formed by two infinite planes, one covered with emissive material and acting as a cathode at zero potential, the other acting as an anode at a potential V0. In the absence of an applieduniform magnetic field directed in the space between the two planes and parallel thereto, the problem is that of Pierce. The electrons go from the cathode to-the anode. If, however, such a uniform magnetic field is directed into the space, the problembecomes quite different. If the field is weak, the electron trajectories are bent perpendicularly to the magnetic field and the planes and curve in, but they still reach the anode. If, however, the field strength is increased to a value which depends on the distance between the two planes and the potential applied between them the electrons no longer reach the anode, but their trajectories are bent until they become tangential to a plane, called the convergence plane between the said two planes and continue therein.

When the cathode emission is limited by space charge the electron trajectories after having reached the plane of convergence remain between that plane and another parallel plane between the same and the cathode. The distance between this new plane and the plane of convergence increases as the space charge increases. Thus the magnetic field, in conjunction with the space charge combine to produce the effect that, in this theoretically conceived infinite diode, the trajectories are focussed or concentrated between two planes parallel to the two elec trodes.

The foregoing is true only in the case of the infinite diode. In the case of a beam emitted .by a limited surface, space charge efiects prevent such focussing. It may be said therefore that the focussing of each finite element in an infinite beam is due in part to the action of electronic charges of the infinite beam at parts exterior to the finite element considered.

The basic principle of the invention consists in providing means having the effect of recreating at the borders of a beam emitted by a cathode, e. g. a plane cathode of finite emissive surface, the conditions of field and of potential such as would occur under otherwise similar conditions if electrons were emitted from a planar cathode .of infinite surface area.

To this end, in carrying out the invention the beam, above supposed infinite, is considered as limited to a part between two trajectories within the supposedly infinite beam acting as limiting trajectories. The equipotential form of the electric field along each trajectory chosen as a limiting trajectory is determined by imagining the whole space to be filled with electron trajectories and by calculation or experiment, ascertaining the form of a number of suitably chosen equi-potentials and, by means of nonmagnetic metal surfaces materializing the equi-potential surfaces at the edges of the beam.

The invention will be better understood upon reference to the accompanying figures in which:

Fig. .1 shows a plane diode with magnetic field, seen as cut by a plane perpendicular to the magnetic field Vector, with the general form of the electron beam; 7

Fig. 2 shows in longitudinal section the general form of an electron gun according to the invention;

Fig. 3 shows in longitudinal section and by way of example an embodiment of the invent-ion in the form of a travelling wave tube within transverse magnetic field, of

plane form, and provided with an electron gun according to the invention;

Fig. 4shows the same tube seen as cut by a plane perpendicular to Fig. 3;

Fig. shows in plan an example of an electronic dis charge device according to the invention adapted to a travelling wave tube of circular form with transverse magnetic field.

Referring to Fig. l, 101 represents in the plane of the figure, a conductive plane maintained at potential 0,

1 covered with an emissive surface, and acting as a cathode.

In the same way, 102 represents a conducting plane maintained at the potential V0 and actingas an anode. 103 and 104 designate two electron trajectories. 105 represents the so-called convergence plane.

The line of thought and calculation which leads to the gun according to the present invention is as follows: First it will be seen and can be shown by calculation, that, if a uniform magnetic field B with lines of force parallel to both conducting planes is introduced in a plane diode such as the one of the figure, and if this field is made to increase starting from null value, the electron trajectories will curve in, and, for a given value of the said field, will become tangential to the anode, which corresponds to the cut oil of the plane magnetron. Then, as the field is increased in intensity the electron trajectories come nearer and nearer to the cathode, while still remaining tangent to the convergence plane and continuing straight on after contact with said plane. This plane becomes nearer and nearer to the cathode as the magnetic field becomes intense. If the cathode emission is limited by space charge, the electron trajectories, after reaching the plane of convergence, remain between this plane and another plane parallel thereto and between it and the cathode. The equations of the electron trajectories passed through before the convergence plane is reached can be determined by the methods of calculation given by Brillouin (Physical Review Sept. l941-p. 385). This method of calculation is valid for an unlimited beam, and gives the trajectories in a system of co-ordinates. The invention provides an electron gun such that although the electron beam is limited to the part included between two trajectories, the elimination of the portions of an unlimited beam, exterior to these trajectories, does not change the form of the remaining trajectories, i. e. the actual trajectories are the same as would be (over the result is achieved by a gun which creates such a system of electric fields that the components of the electric field and the potentials along the electron trajectories remain as though the beam was unlimited, this result being achieved right up to the edges of the beam. The problem i really consists in establishing potential. conditions that,

point of a contour, knowing at the same time the values of V and one ,of its derivatives on the contour. It admits solutions that calculation can give approximately. By means of the so-called electrolytic tank it is possible fairly easily to find the exact form of the equipotentials outside and near the beam. The basic principle employed by the invention consists in providing these equipotentials by surfaces made of non-magnetic metal, and which are given suitable potential.

Figure 2 shows a gun in which 21 is an emissive cathode maintained at the potential V0 and 22 is an electrode maintained at the same potential as the cathode and acting as a control electrode. Transverse magnetic field having lines of force indicated by circles B is produced same lengths) those of a theoretical unlimited beam. This by polar pieces 17 shown in dotted line. In the region near the cathode, the electron flow is subjected to a particular law. Owing to the small velocity of the electrons, the action of the transverse magnetic field is of little effect. In consequence, that electrode makes an angle of 67.5 with the beam, as found by Pierce in the case of the plane diode without any magnetic field. It will be noticed that the cathode is not at the datum potential 0, but at'a negative potential V0 below 0.

After having left the region near the cathode the electrons enter into the region where the action of the magnetic field is important, a region which would correspond to the interaction space of a travelling wave tube. An electrode maintained at the datum potential 0 which may be earth is shown at 23. Near thcbeam, this electrode is shaped to the equipotential form as in the solution of Cauchys problem, and thereafter continues into a plane 31, and thus, conforms with the conducting plane maintained at potential 0 in Fig. 1, so that it provides the necessary transition between the two regions of different conditions between which it extends. Another electrode maintained at a positive potential V2 is shown at 24. The electron beam is indicated at 25. The obtaining of the necessary transition is also assisted by the shaped plate 27 maintained at potential V0 its end 30 being raised in the region of the gun proper where it still takes the form of the equipotential surface, the solution of the Cauchys problem. The part 30 nearly joins the electrode 28 maintained at potential V1 close to V0 but lower than V0, the said electrode 28 ending with an incurving part 29. In the figure, the form of the electron beam 25 is shown, the concentration near the convergence plane being indicated.

Referring to Fig. 3, the foot of the tube is shown at 1; the filament at 2; the cathode at 3, the focussing electrodes at the same potential as the cathode and acting as Wehnelt at 4; the vacuum tight envelope at 5; and the focussing electrode maintained at potential V1 at 6. 7 is an antenna through which an input wave is fed in to a delay line 9 and 12 is an antenna at the output end of the delay line. At 13 is the anode of tube collector, 14 is the insulating piece which holds the cathode potential conductor 8 of the delay line and 15 is the electronic beam.

A B represents the section plane for Fig. 4 which, in section, shows the tube situated between two magnetic pole pieces 17.

The same references designate the same elements as in the previous figure.

Fig. 5 shows in plan, an electron gun according to the invention applied to a travelling wave tube with transverse magnetic field and of rotational cylinder form. The cathode 3, the collector 13 and the beam 15 are situated inside of the envelope 5 of the tube. Both portions 10 and 11 of the control electrode, shaped according to the invention, have been shown joined to the inner part 21 of the tube. The delay line is situated inside the envelope and schematically shown at 9. The junction between the gun parts and the parts defining the interaction space are constituted by the parts 19 and 20 shaped in accordance with the invention. A pair of focussing electrodes 18 also shaped as required by the invention is also represented.

Structures of electron gun according to the present invention have the advantage of giving beams of great intensity-with about 30 times better concentration than guns designed without regard to space charge effects. Since emission is limited by space charge, guns according to this invention are more stable in emission than guns operating at filament saturation, for noise fluctuations and Schottkys etfects are much less. Besides the invention permits of the possibility of varying the flow of the electron beam by modifying the potential of the cathode or of some other auxiliary electrode provided for the purpose. Finally, thanks to the shape of the electrodes used,

smooth junction occurs between the parts of the beam corresponding to two regions of different beam conditions.

emissive cathode capable of emitting an intense stream of electrons, first and second electrodes extending substantially parallel over at least a part of their respective lengths and bounding an interaction space, terminal connections respectively to said cathode and said first and said second electrodes for imparting to said cathode a negative potential with respect to the said first electrode and for imparting to said second electrode a highly positive potential with respect to said first electrode thereby to produce a time constant electric field having its lines of force perpendicular to both said electrodes, means external to said tube for producing a magnetic field having its lines of force perpendicular to said lines of force of said electric field, and an electron gun directing said stream into said space to form a beam the mean direction of which is substantially perpendicular to the lines of force of said magnetic and electric fields, said electron gun comprising a series of at least two profiled members of a non-magnetic metal extending through the portion of space in which said beam is localized before it enters said interaction space, each of said members having an aperture located on the trajecory of said beam for successive passage thereof through said apertures, the surfaces of said members adjacent said apertures being at acute angles with the mean direction of said beam, and terminal connections to said members for imparting different potentials to said members intermediate between said negative potential of said emissive cathode and said highly positive potential of said second electrode.

2. An electron discharge tube according to claim 1, in which the electron gun includes a surface acting as a control or Wehnelt electrode.

3. An electron discharge tube according to claim 1, in which the said first electrode and the said second electrode defining an interaction space are plane and parallel.

4. An electron discharge tube according to claim 1, in which the said first electrode and the said second electrode defining an interaction space are cylindrical and coaxial.

5. An electron discharge tube according to claim 3, in which the said first electrode and the said second electrode defining an interaction space are prolonged on the cathode side by profiled surfaces similar to the said gun surfaces.

6. An electron discharge tube according to claim 4, in which the said first electrode and the said second electrode defining an interaction space are prolonged on the cathode side by profiled surfaces similar to the said gun surfaces.

References Cited in the file of this patent UNITED STATES PATENTS 2,268,194 Glass Dec. 30, 1941 2,511,407 Kleen et al. June 13, 1950 2,530,373 Bowen Nov. 21, 1950 2,531,972 Doehler et al. Nov. 28, 1950 2,567,674. Linder Sept. 11, 1951 2,607,904 Lerbs Aug. 19, 1952 2,680,823 Dohler et al. June 8, 1954 2,694,783 Charles Nov. 16, 1954 OTHER REFERENCES Article by Warnecke et al., pp. 279-291, Annales de Radioelectricite for October 1950. 

